Spread spectrum (SS) communication is presently being used for a number of commercial applications and is expected to proliferate as the demand for untethered communications increases.
An article of A. J. Viterbi titled "When not to spread spectrum-a sequel", which appeared in IEEE Commun. Mag., April 1985, showed Code Division Multiple Access (CDMA) to be inferior in multiple access capability to orthogonal systems such as FDMA and TDMA for a single coverage area. However, in 1990, Gilhousen et al published the paper "Increased Capacity Using CDMA for Mobile Satellite Communication" (IEEE Journal on Selected Areas in Communications, May 1990) which showed that system capacity is increased in a multibeam satellite system by using CDMA with a frequency reuse factor of one (i.e. the total frequency band is reused in each beam) compared to orthogonal systems which would typically employ a frequency reuse of 1:3 or 1:5, depending on the amount of isolation between beams.
In the paper "On the Capacity of a Cellular CDMA System" (IEEE Trans. on Vehicular Technology, May 1991) the capacity advantage of CDMA over orthogonal systems is further investigated for the case of cellular telephone systems. In this cases, the 1:1 frequency reuse of CDMA compares very favorably with the typical 1:7 reuse pattern for analog systems.
The use of "marginal isolation" to provide full frequency reuse in quasi-orthogonal (nonsynchronous) CDMA systems is disclosed in U.S. Pat. No. 4,901,307 to Gilhousen, Jacobs and Weaver, for example.
One example of commercial application of SS is the IS-95 standard for cellular telephones. This system uses Orthogonal CDMA (OCDMA) on the forward (cell-to-mobile) links and nonsynchronous CDMA on the return links. Full frequency reuse is employed in each cell on both forward and return links. Also see U.S. Pat. No. 5,103,459 to Gilhousen et al.
The basic signal format for OCDMA was disclosed by M. J. E. Golay in IDA Report 108, page 110 (1965). The system described by Golay is orthogonal in both the forward and return link directions. One example of an OCDMA system is disclosed in U.S. Pat. No. 5,375,140 assigned to the assignee of the present invention and titled "Wireless Direct Sequence Spread Spectrum Digital Cellular Telephone System" and incorporated herein by reference.
A number of consortiums have been formed to develop satellite based Personal Communications Systems (PCS) with global coverage. These systems include Globalstar (Globalstar System Application before the FCC by Loral Cellular Systems Corp., Jun. 3, 1991), Odyssey (Application of TRW Inc. before the FCC to Construct a New Communications Satellite System "Odyssey," May 31, 1991), Ellipso (filed November, 1990), and ECCO (filed by Constellation Communications Inc. in June, 1991), among others.
The intent of these systems is that a subscriber can place telephone calls directly through the satellite network from almost anywhere on the Earth, using a portable handset much like the present cellular telephones. All of the systems mentioned are required to use spread spectrum CDMA techniques for bandsharing purposes.
The Globalstar application discloses a signal which is essentially the same as the IS-95 standard and the entire frequency band is reused in every beam on both the forward and return links. The other applications, although not so detailed, assume similar waveforms and full frequency reuse.
The forward link of IS-95 differs from the return link in that it uses synchronous orthogonal OCDMA. However, no distinction is made in frequency reuse, and it is assumed that full frequency reuse is the best choice.